Field of the Invention
The present invention relates to a sheet feeder device and an image forming apparatus.
Description of the Related Art
A commercial image forming apparatus such as a production printer is normally expected to cope with various kinds of paper sheets, and there is a demand for image forming apparatuses that are not easily affected by environmental disturbances such as humidity, regardless of thicknesses and types of paper sheets. Particularly, coated paper has a low air permeability and a high moisture absorption rate. Therefore, the sticking force between paper sheets is strong in a high-humidity environment, and multiple feeding or non-feeding often occurs in a sheet feed process.
In a conventional sheet feeder device, to quickly dehumidify the inside of the sheet feed tray, dehumidified/dried air is stored in a storage tank in advance. Immediately after the sheet feed tray is set in the housing, or when a command signal for a start of a sheet feeding operation is received from a control unit, an air blow mechanism blows the dehumidified/dried air from the storage tank into the housing (see JP 2012-150217 A).
However, in the sheet feed tray having paper sheets stacked therein, the sticking force between paper sheets is stronger at lower portions of the sheet stack due to the weight of the paper sheets, and air does not easily enter the sheet stack. As a result, the dryness factor of paper sheets differs between an upper portion and a lower portion of the sheet stack in the sheet feed tray.
FIG. 13 shows changes in weight observed in a case where the temperature and humidity environments for coated paper sheets were changed. The first sheet and the third sheet from the top of a sheet stack, and the sheet in the middle position of the sheet stack (approximately the 250th sheet) were subjected to measurement. First, in the “initial stage”, the weights of the paper sheets were measured in a normal environment. The weights of the paper sheets measured after “moisture absorption” are the results of measurement carried out after the paper sheets were left in an 80% RH environment at 30 degrees centigrade for about 20 hours. The weights of the paper sheets measured “after about 5 minutes” are the results of measurement carried out after the paper sheets having absorbed moisture were moved to a 50% RH environment at 20 degrees centigrade, and were left there for about five minutes.
FIG. 14 shows the rates of change in the weights of the “1st sheet”, the “3rd sheet”, and the “middle sheet” of FIG. 13 between “after moisture absorption” and “after about 5 minutes”. The “middle sheet” in the middle position of the sheet stack has a lower rate of change in the weight of paper and a lower dehumidification rate (dryness factor) than the “1st sheet” and the “3rd sheet”, which are closer to the upper surface of the sheet stack. In this manner, it was confirmed that there was a difference in dehumidification performance between an upper portion and the middle position of the sheet stack. Therefore, when a paper sheet in the middle position is separated from the other paper sheets while the sticking force between the paper sheets having absorbed moisture is still strong, multiple feeding or non-feeding easily occurs.